Microelectronic devices, micromechanical devices, and other devices with microfeatures are typically formed by constructing several layers of components on a workpiece. In the case of microelectronic devices, a plurality of dies are fabricated on a single workpiece, and each die generally includes an integrated circuit and a plurality of bond-pads coupled to the integrated circuit. The dies are separated from each other and packaged to form individual microelectronic devices that can be attached to modules or installed in other products.
One aspect of fabricating and packaging such dies is forming interconnects that electrically couple conductive components located in different layers. In some applications, it may be desirable to form interconnects that extend completely through the dies or through a significant portion of the dies. Such interconnects electrically couple bond-pads or other conductive elements proximate to one side of the dies to conductive elements proximate to the other side of the dies. Through-wafer interconnects, for example, are constructed by forming deep vias on the front side and/or backside of the workpiece and in alignment with corresponding bond-pads at the front side of the workpiece. The vias are often blind vias in that they are closed at one end. The blind vias are then filled with a conductive fill material (e.g., by immersing the workpiece into a solder bath). After further processing, the workpiece can be thinned to reduce the thickness of the final dies. Solder balls or other external electrical contacts are subsequently attached to the through-wafer interconnects at the backside and/or the front side of the workpiece. The solder balls or external contacts can be attached either before or after singulating the dies from the workpiece.
FIG. 1 is a partially schematic side cross-sectional view of a conventional system 10 for depositing solder into openings in a microfeature workpiece. The system 10 can include a chamber 20, a solder bath 30, and a workpiece 40 in the chamber 20 for processing. The solder bath 30 is generally a flat, open reservoir of molten solder or conductive material. The size of the solder bath 30 generally corresponds to the size of the workpiece 40 so that the workpiece can be completely immersed within the solder bath 30 during processing. In one embodiment, for example, the solder bath 30 can include a relatively large and deep container (e.g., about 9 inches by 9 inches by 0.75 inches) filled with AuSn solder (i.e., solder including about 80 percent gold and 20 percent tin). The workpiece 40 can include a plurality of openings or vias (not shown) extending at least partially through the workpiece 40. As discussed below, the openings are at least partially filled with solder from the solder bath 30.
In operation, the workpiece 40 is positioned within the chamber 20 above the solder bath 30 and air or other gases within the chamber 20 are exhausted to create a vacuum within the chamber 20. The workpiece 40 is then at least partially immersed into the solder bath 30 (as shown in broken lines). The chamber 20 is then pressurized to a desired pressure (e.g., using nitrogen (N2) gas) and the differential pressure between the inside of the chamber 20 and the inside of the openings within the workpiece 40 causes the solder in the solder bath 30 to be sucked into the openings in the workpiece 40. The workpiece 40 is then removed from the solder bath 30 and cooled. The workpiece 40 can then be removed from the chamber 20 for further processing.
Conventional systems for depositing conductive material into openings in workpieces, such as the solder bath 30 of the system 10, include several drawbacks. One drawback with the system 10 is that it can be very expensive to keep the solder bath 30 full of solder. In embodiments using AuSn solder, for example, it can cost well over $100,000 to keep an adequate volume of solder in the solder bath 30 for processing the workpiece 40. Because the openings in the workpiece 40 are extremely small and filling the openings requires very little solder material, much of the solder within the solder bath 30 can go to waste after processing.
Another drawback with the conventional approach described above is the large exposed surface area of the workpiece 40 as the workpiece is removed from the solder bath 30. Dross begins to form on the workpiece 40 almost immediately after the workpiece 40 is removed from the solder bath 30. This oxidation can require additional processing steps for removal and/or cause contamination or defects in the workpiece 40. Still another drawback with the conventional approach described above is that the workpiece 40 is relatively buoyant within the solder bath 30 and it can be difficult to completely cover the workpiece 40 with solder. As a result, the solder may not completely fill the openings and/or the solder may not be distributed uniformly across the workpiece 40. Accordingly, there is a need to improve the system and methods for depositing solder or other conductive materials into openings in microfeature workpieces.